There are several pressure types to consider in choosing pressure transmitter and pressure sensor, but they can all be divided into two broad categories—vented and sealed.

Vented pressure types use atmospheric pressure as the reference point, or the zero point. By definition, vented sensors are not affected by barometric or elevation changes as they are vented to atmosphere. The reference point becomes whatever the barometric pressure is at the time, and is therefore immune to these changes.

The vented category includes:

Gage: Gage sensors begin at zero and measure pressure up to their rated full-scale pressure range.

Vacuum: Vacuum sensors measure from zero to a full vacuum, not exceeding -14.75 PSI.

Compound: Compound pressure sensors measure from full vacuum pressure to the full-scale pressure range.

Sealed sensors have a sealed reference point that everything is measured against. As such, they are influenced by changes in barometric pressure and elevation. The zero point of these sensors is the pressure under which the sensors were sealed.

The two sealed pressure types are:

Absolute: An absolute sensor is sealed under a full vacuum, which becomes its zero point.

Sealed: A sealed sensor is sealed under whatever the atmospheric pressure was at the time. This is usually done to protect the sensor from moisture and dust.

The higher the pressure range, the less impact this decision will have on sensor performance. For example, at 5,000 PSI, the approximate 15 PSI difference between an absolute sensor and a gage sensor is usually insignificant.

I would suggest the pressure type be considered very carefully in any pressure range below 1,000 PSI, as the small changes in atmospheric pressure become a greater percentage of full-scale. The questions to ask are:

Where do I want my zero reference point to be?

Do I want it to change with barometric pressure?